U.S. patent number 5,712,731 [Application Number 08/549,681] was granted by the patent office on 1998-01-27 for security device for security documents such as bank notes and credit cards.
This patent grant is currently assigned to Thomas De La Rue Limited. Invention is credited to Kenneth J. Drinkwater, Philip M. G. Hudson.
United States Patent |
5,712,731 |
Drinkwater , et al. |
January 27, 1998 |
Security device for security documents such as bank notes and
credit cards
Abstract
A security device includes an array of microimages which, when
viewed through a corresponding array of substantially spherical
microlenses, generates a magnified image. In some cases, the array
of microlenses is bonded to the array of microimages.
Inventors: |
Drinkwater; Kenneth J. (Surrey,
GB), Hudson; Philip M. G. (Wiltshire, GB) |
Assignee: |
Thomas De La Rue Limited
(London, GB)
|
Family
ID: |
10735268 |
Appl.
No.: |
08/549,681 |
Filed: |
December 12, 1995 |
PCT
Filed: |
May 10, 1994 |
PCT No.: |
PCT/GB94/01006 |
371
Date: |
January 02, 1996 |
102(e)
Date: |
January 02, 1996 |
PCT
Pub. No.: |
WO94/27254 |
PCT
Pub. Date: |
November 24, 1994 |
Foreign Application Priority Data
|
|
|
|
|
May 11, 1993 [GB] |
|
|
9309673 |
|
Current U.S.
Class: |
359/619; 359/622;
359/623 |
Current CPC
Class: |
G02B
5/1885 (20130101); G03H 1/0011 (20130101); G06K
19/14 (20130101); G06K 19/16 (20130101); G02B
3/0056 (20130101); G02B 3/0075 (20130101); G03H
1/0244 (20130101); G03H 2001/221 (20130101); G03H
2210/55 (20130101); G03H 2223/19 (20130101) |
Current International
Class: |
G06K
19/14 (20060101); G06K 19/16 (20060101); G02B
027/10 () |
Field of
Search: |
;359/619,626,622,623 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
203752 |
|
Dec 1986 |
|
EP |
|
215672 |
|
Mar 1987 |
|
EP |
|
Primary Examiner: Nelms; David C.
Assistant Examiner: Mack; Ricky
Attorney, Agent or Firm: Oliff & Berridge, P.L.C.
Claims
We claim:
1. A security device comprising a regular two dimensional array of
substantially identical printed microimages, each of the
microimages having a dimension of up to about 250 .mu.m and which,
when viewed through a two dimensional array of substantially
spherical microlenses having substantially a same pitch as the
array of microimages, each microlens having a diameter in the range
of substantially 50-250 .mu.m, generate at least one magnified
version of one of the microimages when the array of microimages and
the array of microlenses are at least partially registered.
2. A security device according to claim 1, wherein the microimages
have been printed with ink.
3. A security device according to claim 2, wherein the microimages
have been lithographically or intaglio printed.
4. A security device according to claim 1, further comprising a two
dimensional array of substantially spherical microlenses having
substantially a same pitch as the array of microimages, each of the
microlenses having a diameter in the range of substantially 50-250
.mu.m secured over the array of microimages to generate at least
one magnified version of one of the microimages when the array of
microimages and the array of microlenses are at least partially
registered.
5. A security device according to claim 4, wherein the array of
microlenses is formed in a transparent polymeric composition.
6. A security device according to claim 5, wherein the transparent
polymeric array composition is colored.
7. A security device according to claim 6, wherein the microlens
array is transparent and provided with stripes of different
colors.
8. A security device according to claim 1, wherein the array of
microimages includes a plurality of sets of microimages, and
wherein the microimages within each of the sets are the same while
the microimages of each of the sets differ from the microimages of
other ones of the sets.
9. A security device according to claim 8, wherein different ones
of the microimages define different views of a same image.
10. A security device according to claim 8, further comprising a
two-dimensional array of substantially spherical microlenses having
substantially a same pitch as the array of microimages, each of the
microlenses having a diameter in the range of substantially 50-250
.mu.m secured over the array of microimages to generate at least
one magnified version of one of the microimages when the array of
microimages and the array of microlenses are at least partially
registered, wherein the array of microimages includes at least two
sets of microimages, at least one of the sets of microimages having
a pitch different from a pitch of the microlens array such that
each magnified version of one of the microimages includes at least
two elements moving at different apparent rates and forming
different numbers magnified images of each element.
11. A device according to claim 8, wherein each of the sets of
microimages is differently colored.
12. A security device according to claim 1, wherein the microlenses
comprise astigmatic spherical lenses having different focal lengths
in mutually orthogonal directions around the optical axis.
13. A security device comprising an optical device exhibiting an
optically variable image, the optically variable device further
including a two dimensional array of substantially identical
microimages, each of the microimages having a dimension of up to
about 250 .mu.m and which, when viewed through a two dimensional
array of substantially spherical microlenses having substantially a
same pitch as the array of microimages, each microlens having a
diameter in the range of substantially 50-250 .mu.m, generate at
least one magnified version of one of the microimages when the
array of microimages and the array of microlenses are at least
partially registered.
14. A security device according to claim 13, wherein the optically
variable image comprises a hologram.
15. A security device according to claim 14, wherein the hologram
is formed by a surface relief pattern.
16. A security device according to claim 13, wherein the
microimages are provided as a surface relief.
17. A security device according to claim 13, further comprising a
two dimensional array of substantially spherical microlenses having
substantially a same pitch as the array of microimages, each of the
microlenses having a diameter in the range of substantially 50-250
.mu.m secured over the array of microimages to generate at least
one magnified version of one of the microimages when the array of
microimages and the array of microlenses are at least partially
registered.
18. A security device according to claim 17, wherein the array of
microlenses is formed in a transparent polymeric composition.
19. A security device according to claim 18, wherein the
transparent polymeric composition is colored.
20. A security device according to claim 19, wherein the microlens
array is transparent and provided with stripes of different
colors.
21. A security device according to claim 17, comprising a
transparent substrate on one side of which is provided the array of
microlenses and on the other side of which is provided the array of
microimages.
22. A security device according to claim 21, wherein the
transparent substrate is plastics.
23. A security device according to claim 13, wherein the
microlenses comprise astigmatic spherical lenses having different
focal lengths in mutually orthogonal directions around the optical
axis.
24. A security device according to claim 13, wherein the array of
microimages includes a plurality of sets of microimages, and
wherein the microimages within each of the sets are the same while
the microimages of each of the sets differ from the microimages of
other ones of the sets.
25. A security device according to claim 24, wherein the different
ones of the microimages define different views of a same image.
26. A security device according to claim 24, further comprising a
two-dimensional array of substantially spherical microlenses having
substantially a same pitch as the array of microimages, each of the
microlenses having a diameter in the range of substantially 50-250
.mu.m secured over the array of microimages to generate at least
one magnified version of one of the microimages when the array of
microimages and the array of microlenses are at least partially
registered, wherein the array of microimages includes at least two
sets of microimages, at least one of the sets of microimages having
a pitch different from a pitch of the microlens array such that
each magnified version of one of the microimages includes at least
two elements moving at different apparent rates and forming
different numbers of magnified images of each element.
27. A security device according to claim 24, further comprising a
two-dimensional array of substantially spherical microlenses having
substantially a same pitch as the array of microimages, each of the
microlenses having a diameter in the range of substantially 50-250
.mu.m secured over the array of microimages to generate at least
one magnified version of one of the microimages when the array of
microimages and the array of microlenses are at least partially
registered, wherein the array of microimages includes at least two
sets of microimages, at least one of the sets of microimages having
a pitch different from a pitch of the microlens array such that
each magnified version of one of the microimages includes at least
two elements moving at different apparent rates and forming
different numbers of magnified images of each element.
28. A method of preparing a record medium for use in producing a
security device, the method comprising imaging an object or image
onto the record medium through a regular two dimensional array of
substantially spherical microlenses, each of the microlens having a
diameter in the range of substantially 50-250 .mu.m, so as to
record a two dimensional array of substantially identical
microimages, each of the microimages having a dimension of up to
about 250 .mu.m and having substantially a same pitch as a pitch of
the array of microlenses.
29. A method according to claim 28, wherein the record medium
comprises a printing plate.
30. A method according to claim 28, wherein the imaging step
comprises imaging the object or image onto a record medium through
a plurality of different two dimensional arrays of substantially
spherical microlenses, each of the microlens having a diameter in
the range of substantially 50-250 .mu.m.
31. A method according to claim 30, wherein the image or object is
imaged through two of said arrays of substantially spherical
microlenses.
32. A method according to claim 30, wherein pitches of the arrays
of microlens vary by between 5 and 50%.
33. A method according to claim 32, wherein the variation in the
pitches is substantially 20%.
34. A method of manufacturing a security device, the method
comprising:
printing a regular two dimensional array of substantially identical
microimages on a record medium, each of the microimages having a
dimension up to about 250 .mu.m; and
securing over the array of microimages, a two dimensional array of
substantially spherical microlenses having substantially a same
pitch as the array of microimages, each of the microlens having a
diameter in the range of substantially 50-250 .mu.m, such that on
viewing, at least one magnified version of one of the microimages
is visible when the array of microimages and the array of
microlenses are at least partially registered.
35. A method according to claim 34, wherein an allowable tolerance
during the securing step between the array of microimages and the
array of microlenses is in a range of between 0.25-0.5 mm.
36. A method according to claim 34, wherein the printing step
comprises a lithographic or intaglio process.
37. A method according to claim 34, wherein step of securing an
array of substantially spherical microlenses comprises laying a
transparent medium on the record medium, and forming the array of
microlenses in the transparent medium.
38. A method according to claim 37, wherein the forming step
comprises an intaglio process.
39. A security document carrying a security device, the security
device comprising a regular two dimensional array of substantially
identical printed microimages, each of the microimages having a
dimension of up to about 250 .mu.m and which, When viewed through a
two dimensional array of substantially spherical microlenses having
substantially a same pitch as the array of microimages, each
microlens having a diameter in the range of substantially 50-250
.mu.m, generate at least one magnified version of one of the
microimages when the array of microimages and the array of
microlenses are at least martially registered.
40. A security document carrying a security device, the security
device comprising an optically variable device exhibiting an
optically variable image, the optically variable device further
including a two dimensional array of substantially identical
microimages, each of the microimages having a dimension of up to
about 250 .mu.m and which, when viewed through a two dimensional
array of substantially spherical microlenses having substantially a
same pitch as the array of microimages, each microlens having a
diameter in the range of substantially 50-250 .mu.m, generate at
least one magnified version of one of the microimages when the
array of microimages and the array of microlenses are at least
partially registered.
41. A security document carrying a security device, the security
device comprising a record medium, the record medium being prepared
by a method comprising imaging an object or image onto the record
medium through a regular two dimensional array of substantially
spherical microlenses, each of the microlens having a diameter in
the range of substantially 50-250 .mu.m, so as to record a two
dimensional array of substantially identical microimages, each of
the microimages having a dimension of up to about 250 .mu.m and
having substantially a same pitch as a pitch of the array of
microlenses.
42. A security document carrying a security device, the security
device being manufactured by a method comprising:
printing a regular two dimensional array of substantially identical
microimages on a record medium, each of the microimages having a
dimension up to about 230 .mu.m; and
securing over the array of microimages, a two dimensional array of
substantially spherical microlenses having substantially a same
pitch as the array of microimages, each of the microlens having a
diameter in the range of substantially 50-250 .mu.m, such that on
viewing, at least one magnified version of one of the microimages
is visible when the array of microimages and the array of
microlenses are at least partially registered.
43. A method of authenticating a security device, the security
device including a regular two dimensional array of substantially
identical printed microimages, each of the microimages having a
dimension of up to about 250 .mu.m, the method comprising:
positioning a two dimensional array of substantially spherical
microlenses having substantially a same pitch as a pitch of the
array of microimages, each of the microlenses having a diameter in
the range of substantially 50-250 .mu.m, over the security device;
and
viewing the security device through the array of microlenses.
44. A method of authenticating a security device, the wherein
security device an optically variable device exhibiting an
optically variable image, the optically variable device further
including a two dimensional array of substantially identical
microimages, each of the microimages having a dimension of up to
about 250 .mu.m, the method comprising:
positioning a two dimensional array of substantially spherical
microlenses having substantially a same pitch as a pitch of the
array of microimages, each of the microlenses having a diameter in
the range of substantially 50-250 .mu.m, over the security device;
and
viewing the security device through the array of microlenses.
45. A security device reader comprising:
a two dimensional array of substantially spherical microlenses,
each of the microlenses having a diameter in the range
substantially 50-250 .mu.m; and
a security document holder for locating a security document having
a security device, the security device comprising a regular two
dimensional array of substantially identical printed microimages,
each of the microimages having a dimension of up to about 250 .mu.m
and which, when viewed through the two dimensional array of
substantially spherical microlenses having substantially a same
pitch as the array of microimages, generate at least one magnified
version of one of the microimages when the array of microimages and
the array of microlenses are at least partially registered.
46. A reader according to claim 45, wherein the array of
microlenses and the security document holder are relatively
rotatable, the axis of rotation being normal to a plane defined by
the array of microlenses.
Description
The invention relates to a security device and methods of making
such security devices.
BACKGROUND OF THE INVENTION
Many different proposals have been made in the past for designing
security devices for affixing to security documents such as
banknotes, credit cards and the like to assist in authenticating
such security documents during their use. Typical examples, include
holograms and other optical variable devices.
One particular type of security device which has been used for many
years is the security thread. Recently, in U.S. Pat. No. 4,892,336,
a development of the security thread device was described. In this
case, the security thread was provided as a transparent substrate,
one side of which carried a set of lenses and the other side of
which carried microprinting which could be viewed through the
lenses. Typically, the microprinting comprises strips of different
colours such that when the thread is viewed at different angles,
different colours will be perceived. One of the problems with this
approach is the need for a very precise register between the
microlenses and printing. In fact, in U.S. Pat. No. 4,892,336 this
need for precise register is put forward as one of the advantages
of that invention in that it makes it very much more difficult to
counterfeit such security devices. On the other hand, for a
security device to be useful commercially, genuine devices must be
relatively easy to manufacture since otherwise production costs
will be prohibitive.
U.S. Pat. No. 4,765,656 also describes a security device made using
a lenticular screen and in this case the microimages are formed by
direct laser writing through the microlenses which are already in
situ in the device. Again, this approach is not suited to mass
production techniques although it does achieve exact register
between the lenses and images.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a security
device comprises a regular two dimensional array of substantially
identical printed microimages, each having a dimension up to about
250 .mu.m and which, when viewed through a two dimensional array of
substantially spherical microlenses having substantially the same
pitch as the microimage array, each microlens having a diameter in
the range substantially 50-250 .mu.m, generate one or a number of
magnified versions of the microimage depending on the extent to
which the array of microimages and the array of microlenses are in
register.
We have realised that a new security device can be constructed
which is suited to mass production techniques by utilising the
effects of a two dimensional array of spherical microlenses.
Generally, each microlens will have identical optical properties.
The viewing condition also assumes that the microimage objects are
situated near the focal plane of the microlenses. In a first
"ideal" example of this system, the pitches of the images and
lenses are identical and the two are accurately aligned and the
microimages are near the focal planes of the microlenses. When this
system is viewed from a given direction each lens images the
corresponding point on the image underneath it to form a very
highly magnified image of the underlying microimage element. If
there is a difference in pitch between the two arrays a set of
moire fringes will appear where the repeat element is the object in
the array when across the moire fringe successive elements image
successive portions of the object to build up an image which will
repeat every time the mismatch equals an integer number of lenses.
A pitch mismatch between a microlens array and microimage array can
also conveniently be generated by angularly misaligning the arrays
which will also generate a moire repeating image of the object. It
should be appreciated that most of the practical embodiments of
this invention will use the misaligned condition of the arrays to
form images as this arrangement has particularly advantageous
properties.
The term spherical lenses means that the surface form of each
microlens approximates to a portion of the surface of a sphere such
that the lens has imaging power along two perpendicular axes around
its optic axis (which will be normal to the plane of the array). In
this case, the lens surface will be curved in both axes
perpendicular to the optic axis with both axes having the same
curvature. In this case, the lens will form an approximate point
image at its focal plane when illuminated with collimated light
(neglecting aberrations and small deviations from ideal behaviour
due to small deviations of the shape of the lens from the ideal
spherical shape). Effectively spherical lenses are the normal form
of simple lens which will form a normal image of an object near the
focal plane. This is in contrast to cylindrical lenses which only
have optical power in one axis and would therefore form a line
image when illuminated with collimated light with the light being
brought to a focus in one axis but not in the other and in this
case the surface profile of the lens would be a portion of the
surface of a cylinder with a radius of curvature in one, direction
and flat (linear) in a perpendicular direction. By the term
astigmatic lens we mean a lens surface profile that is very similar
to that of a spherical lens but where the radius of curvature along
the two perpendicular optical axes is different such that the lens
forms a distorted image ,of an object by bringing different
perpendicular components of light to focus at different distances.
An astigmatic lens illuminated by collimated light would focus the
light at different distances in both axes.
Each array would normally consist of many lenses and microimage
elements, repeating many times in each direction with a regular
pitch. Typically such arrays consist of many (e.g. 100 or more)
regularly spaced identical lenses, each typically 50-250 .mu.m in
diameter and with a focal length of typically 200 .mu.m. A typical
manufacturing process would be to coat a substrate with
photoresist, expose this to a mask say a grid or hexagonal pattern)
and then develop off the desired areas to leave isolated, regular
islands of resist. These are then melted and surface tension in the
liquid state causes the formation of an array of spherical islands
of resist. From this a microscopic array of spherical structures
can be manufactured by known replication processes.
In particular, in contrast to the prior art approaches, many
examples of the security device will not include the microlens
array. This makes it much easier and cheaper to manufacture than
the known devices which incorporate microlens arrays. Also, in
contrast to the prior art, we have realised that by using spherical
lenses, the need for exact register between the lens array and the
microimage array is relaxed which makes systems of combined lens
arrays and microimages much easier to manufacture and particularly
allows for a manufacturing method utilising established security
printing techniques.
Typically, the microimages will have feature sizes down to say 5 or
10 microns. This together with their small size makes the
microimages very difficult to reproduce and in particular difficult
to see by the naked eye. Consequently, this security device is
covert but easy to use.
In some cases the array of microimages will form a single set of
substantially identical microimages while in others several such
sets are provided, the images of each set being identical with
others in the same set but different from other sets. In this
latter case, the sets could record different views of an object
thus providing a 3D effect when viewed.
The interaction between the analysing array of microlenses and a
corresponding set of identical microimages is of a characteristic
visual form. When the corresponding arrays are perfectly aligned
into register each lens has underneath it-a microimage in perfect
register so that an observer sees only one magnified image of the
microimages. However, as the spherical lens array is rotated
relative to the image array the single magnified image splits into
a regular array of images with the number of visible images
increasing and their individual sizes decreasing as the angular
mismatch increases. Essentially on twisting the lens/image arrays
off the location of perfect register, the area and unit length over
which the lenses and images are in phase to form a visible picture
decreases so that the device ceases to display a single image and
starts to display an array of much smaller regular images which
show the loss of perfect register between the lenses and the
microimages. The overlap image areas are essentially determined by
the moire pattern formed between the lens and image arrays and so
this device has been termed the "moiremagnifier". It is this
"moire"characteristic of the device that means much looser
register/alignment constraints are required to form a viewable
image than for instance would be the case with previous known
systems based on cylindrical lens arrays held in perfect register
with micro printing where the image is destroyed when register and
tilt alignment are lost. It is anticipated that the majority of
security devices would use this form of array of identical or
repeated micro images to allow relative positional register
tolerances during manufacture to be kept very loose.
In an alternative approach, three-dimensional objects may be
recorded with each microimage corresponding to a slightly different
view point of the object so that when the microimages are viewed
through the corresponding matched lens array positioned in exact
register, depth/movement effects will be visible as the viewer
changes viewpoint. Note that this 3D image will be pseudoscopic but
that techniques exist for reversing the recorded images, for
example using retroreflectors, to produce orthoscopic (correct
parallax) images.
These depth and movement effects across the image can be used to
provide additional effects. The ultimate case becomes a complete
"integral photograph". This would be, for example, recorded through
a microlens array using a 3D subject (or several spaced apart
planar graphical images) to produce an image. Typically at these
small sizes the image would not be visible to the unaided eye.
Typically such images would involve recording small scale detail,
probably down to 5-10.mu. in width.
However, in the case of a complete integral photograph, perfect
alignment is required between the lens array and the corresponding
images to view the result and the image would not be visible
without this. Therefore, although this type of image is possible it
is anticipated that this form of device is unlikely to be used for
printed or mass produced security devices because the register
tolerances for image formation would be much higher than for the
case of repeated simpler microimages where moireeffects would occur
on misalignment allowing much looser register constraints for image
visibility.
It will be appreciated that an important aspect of this invention
is that the microimages are printed, e.g. with ink. This makes the
device particularly suitable for mass production.
In accordance with a second aspect of the present invention, a
method of preparing a record medium for use in producing a security
device comprises imaging an object or image onto a record medium
through a regular two dimensional array of substantially spherical
microlenses, each microlens having a diameter in the range
substantially 50-250 .mu.m, so as to record a two dimensional array
of substantially identical microimages, each having a dimension of
up to about 250 .mu.m and substantially the same pitch as the
microlens array. The record medium may itself constitute or can
form a printing plate or it can be used to replicate the resultant,
recorded microimages onto a printing plate.
In one example a microlens array forms a set of microimages on a
material which is then used to form a printing plate. This pattern
is then printed onto a document as an array of tiny structures, of
a size just above that of print resolution e.g. 5 to 10.mu. up to
about 100.mu. but which are not individually visually discernible.
This array could then subsequently be decoded using a microlens
array similar to that used originally in manufacture. The creation
of such a precisely spaced set of microimages would be very
difficult using current techniques such as engraving and photo
techniques. Yet certain print techniques themselves such as
lithography or intaglio, can resolve down to around 5.mu. to enable
such a structure to be printed from a printing plate.
These developments therefore relate to the creation of new
microimaged security features which may be printed. In all cases
the microimages will be viewed through matched, corresponding
microlens arrays and in all cases either the viewing itself or more
sophisticated multiple image or lenticular techniques can produce
visually distinct movement, depth and image "switching" i.e. abrupt
change of the perceived image as a result of a change of the
viewing angle, effects.
In other examples, an object could be imaged onto a substrate
carrying photoresist which is then processed in a conventional
manner to generate a printing plate.
In most of the examples described so far, the security device does
not include a corresponding array of microlenses. However, in
accordance with a third aspect of the present invention, a method
of manufacturing a security device comprises printing a regular two
dimensional array of substantially identical microimages on a
record medium, each having a dimension up to about 250 .mu.m; and
securing over the array of microimages, a two dimensional array of
substantially spherical microlenses having substantially the same
pitch as the microimage array, each microlens having a diameter in
the range substantially 50-250 .mu.m, such that on viewing, one or
a number of magnified versions of the microimage, depending on the
extent to which the array of microimages and the array of
microlenses are in register, is seen.
In this case, a security device is provided comprising combined
microimages and microlenses. However, this method is particularly
suited to mass production since the tolerance with which the
microlenses need to be registered with the microimages is much
wider than has previously been possible. Typically, the allowable
tolerance during the securing step between the arrays of
microimages and microlenses is in the range 0.25-0.5 mm.
In this case, the printed microimages and the viewing microlens
array are integrated together as a public recognition security
feature which would produce an approximation of the kind of
optically variable effects, depth perception effects, parallax
movement effects and rapid holographic image changes associated
with white light interfering structures such as multiple
alternating layers of metal, dielectric coatings, or white light
diffracting effects such as are exhibited by surface relief pattern
forming holograms and computer generate diffraction patterns such
as the Kinegram of Landis and Gyr.
A particularly important application of the invention provides a
security device in accordance with a fourth aspect of the invention
comprising an optically variable device exhibiting an optically
variable image, the device further including a two dimensional
array of substantially identical microimages each having a
dimension up to about 250.mu.m and which, when viewed through a two
dimensional array of substantially spherical microlenses having
substantially the same pitch as the microimage array, each
microlens having a diameter in the range substantially 50-250
.mu.m, generate one or a number of magnified versions of the
microimage depending on the extent to which the array of
microimages and the array of microlenses are in register.
Typically, the optically variable image will comprise a hologram or
diffraction grating, for example a surface relief hologram or a
diffraction grating structure such as a Kinegram or a "pixelgram"
as made by CSIRO. In general, the microimages will either be
printed onto an optically variable device layer or provided as a
light diffracting surface relief pattern.
This aspect of the invention involves combining with a standard
optically variable device such as a security embossed hologram, an
array of tiny integral images e.g. photographs, made and read out
using the microlens structures and techniques outlined above.
The gross holographic image thereby provides a first level of
public recognition, and thus security.
In use, the optically variable device, such as a hologram, would
provide security through being instantly publicly recognisable
whilst the small scale microscopic integral images would not be
discernible by eye, but would act as a covert feature becoming
visible when overlaid by a microlens array (supplied as an embossed
plastic film) to reveal the hidden images whether they are of text,
graphics or code markings.
This is useful as a security feature because the very small size
attainable in integral photographs and by imaging through microlens
arrays is very difficult to obtain by printing or photography. It
is totally beyond the capability of normal reprographic scanning or
printing methods. It is also very difficult to produce entirely
holographically. It is too fine for conventional artwork, and
anyone succeeding in producing such artwork would be defeated in
the holography as the precise imaging of such structures would be
extraordinarily difficult if not impossible and such fine lines
could not be recorded by masking techniques.
The only way to obtain such a device would therefore be to obtain
and use a microlens array identical to that used for manufacture.
This is therefore a valuable security device inaccessible to both
unauthorised holographers and unauthorised printers.
The hologram and microscopic integral images e.g. photographs would
be produced together on a first master embossing plate as detailed
below and would then be mass produced by conventional embossed
hologram production techniques.
An analysing "viewer" sheet which consists of an array of surface
relief microlenses would also be manufactured by embossing or
possibly moulding. These viewers could then be distributed to
authorised users to allow definitive verification of the
hologram.
Preferably in this technique the integral photography stage is
performed by imaging down some two dimensional appearance graphics
onto a photoresist or other recording medium although this first
pattern could also be produced using semiconductor direct writing
photomask manufacturing methods. The plate on which the integral
photograph is recorded would also be used to record either prior or
subsequently a conventional photoresist rainbow hologram or
diffraction grating type device as known in the art. This would
then combine both the holographic and integral microimages e.g.
photographic images into the same surface relief structure. The
recombination could also be done by mechanical stamping from
individual masters. Note that although we use the term "integral
photography" the images recorded are two dimensional and actually
much smaller in scale than conventional integral photographs
because of the small scale size of the images produced here due to
the availability of very small scale lens arrays.
At the origination stage the preferred production technique would
consist of two stages in any order:
A. Record standard hologram into photoresist.
B. Put microlens array over photoresist at a precise spacing and
record a holographic image of the image graphics/logo/3D
object.
The photoresist would then be developed.
The microimages would be recorded into areas containing a
holographic replay (e.g., diffraction grating, rainbow hologram,
2D/3D hologram or a matt speckled, diffused patch). This would
enable holograms to be manufactured containing both standard
diffractive structures whose surface relief pattern may have a
resolution of around 1 micron and microscopic integral photographs
of diameter 10-100.mu. as distinct areas on the holographic
surface. It is also possible to produce similar structures using
imaging or other techniques to record the 2D integral photographs
as small scale holographic structures in their own right.
The secondary image produced and revealed by the microlens array
could take several forms made as follows:
1. A 2D microimage is made by holographically recording a flat, two
dimensional graphic image through the lens array. On viewing, a
repeated set of these graphics would be visible: these would show
movement effects.
2. A microimage showing rapid image switching effects as a function
of viewing angle, is made by holographically recording several sets
of flat graphical artwork through the lens array, at different
recording angles. On tilting the device a viewer would see sharp
switches between various graphics as alternative images came into
view.
3. A full three dimensional integral "photograph" is made where the
object being holographically recorded is three dimensional.
Different microlenses record views from different directions of
parallax. When the decoding microlens array is in perfect alignment
a viewer would see a three dimensional image exhibiting parallax
shifts and depth effects on tilting.
The analysing microlens could be a lens structure embossed into a
transparent plastic film as discussed. There may also be advantages
in using a holographic microlens as the analyser.
This aspect of the invention could be used as a supplementary level
of covert security on banknote holograms or other optically
variable devices, or plastic cards.
Security devices in which the microimage arrays are printed down
onto a document and then the microlens array is formed or fixed on
top of the microimage array in order to form a permanently bonded
public recognition security feature are particularly appropriate
for protecting documents of value for example bonds, cheques,
banknotes or plastic cards against counterfeit as the image
features are optically variable (e.g. image switches, colour
shifts) and the devices and image characteristics cannot be
counterfeited or copied using routinely available printing
technology such as colour copiers or scanners.
In order to view a security device which does not already
incorporate a microlens array, we provide in accordance with a
fifth aspect of the present invention a method of authenticating a
security device comprising positioning a two dimensional array of
substantially spherical microlenses having substantially the same
pitch as the microimage array, each microlens having a diameter in
the range substantially 50-250 82 m, over the security device, and
viewing the security device through the array of microlenses.
The security device would be viewed through the corresponding array
of microlenses in order to observe an image. A suitable array of
spherical microlenses comprises a clear plastic sheet with the
lenses incorporated by roll or flatbed embossing, injection
moulding, vacuum forming casting and via curing in a mould as
appropriate. In order to verify the security device the array of
microlenses would simply be laid on top of the printed or
holographic device and rotated. As the lens array and microimage
array come into register an array of small identical images would
be observed due to the moire phenomenon between the arrays. On
perfect alignment a single enlarged magnified image would become
visible.
This can be done manually but conveniently, we provide in
accordance with a sixth aspect of the invention a security device
reader comprising a two dimensional array of substantially
spherical microlenses having substantially the same pitch as the
microimage array, each microlens having a diameter in the range
substantially 50-250 .mu.m; and a security document holder for
locating a security document having a security device substantially
in register with the array of microlenses such that the security
device can be viewed through the microlenses. Typically, the holder
and microlens array would be relatively rotatable. The changing
arrays of identical images seen as the two arrays are rotated would
form a highly characteristic visual security feature.
By using more than one set of microimages, this invention enables
an optically variable image switching effect to be achieved using
much simpler systems than previously. Consider the case where a set
of micro lenses is used to record two arrays of images of objects A
and B. The two objects are angularly separated and so record
microimages that will only become visible at different view angles.
Now suppose the resultant image array is combined with an overlaid
lens array to form a security device. The image characteristics of
this security device will be that on tilting the device the viewer
would see the observed image flip between the two sets of recorded
information A or B or multiple images of these depending on the
moire interaction between the images and arrays. In the case of a
printed document the microimage containing microimages A and B
would be printed down at one working, so keeping the exact relative
registration between the two microimages and so keeping a defined
angular separation and shift. This system could of course cover the
case of more than two images recorded, for example several sets of
images replaying at different angles used to a progression such as
several rings to form an expanding circle pattern or to incorporate
simple image movement or progression forming effects similar to
those frequently used in multiplex holography where in this case
the angular selectivity of the microlens array is being used to
encode the various viewing directions to produce image "switches"
rather than a hologram. It is known that such obvious image
switches produce effective public recognition security devices and
are significantly more complex to produce thus providing
significantly increased security. This is described in more detail
below in connection with FIG. 2.
The simultaneous viewing of two or more sets of different
microimage arrays via a lens array can also be used to form a
device of colour shifting appearance. Suppose for example the two
different microimage sets A and B as above are separately recorded
and then each printed separately onto a document in visibly
different coloured inks. The security device would then be formed
by overlaying this microimage set with a set of matched spherical
lenses. In this case a viewer would see on tilting the device an
image switch and a predefined colour switch. The angular image
switches between different devices would vary because using
standard printing techniques the two different coloured microlens
arrays would not retain the same relative register device to
device. However, the colour shift on switch between the two images
would be well defined and constant in all security devices. In this
way colour switching security devices could be formed without the
need for exact in register print as for cylindrical lenses. The
device would simply be defined to operate in a mode utilising the
twist error moire effect to produce an array of a few multiple
images switching between images of different colour on tilting so
that different devices within a manufacturing run would look
substantially similar.
In some cases, particularly where the corresponding microlens array
is not part of the security device, it is important to prevent the
microimage arrays from being back engineered or to make it very
difficult to back engineer from the analysing lens arrays that
would be distributed in the field as readers.
One preferred method is to use two different lens arrays of related
pitches when recording the initial image deliberately chosen to
have similar but different pitches, for example, varying by for
example 20% (range 5%-50%). The initial array would be recorded by
imaging two separate images, say, A and B each through different
lens arrays, one for A and one for B, into the same recording
material. To form a printed security device the resultant pattern
would then be printed onto a document or otherwise reproduced and
then overlaid with a microlens array with a suitable twist offset
to produce several multiple images by the moire effect. The
resultant security device would then display a number of images A,
the number and pitch set by the moire interaction between the
microimages and analysing array plus an array of images B with a
different size, number and pitch depending on the different moire
interaction between this microimage array and the corresponding
analysing lens array. This image consisting of two different moire
generated repeating patterns would be very difficult to back
engineer as two lens sets would be required. The analysing lens
system could be similar to either of the recording lenses.
Depending on the relative periodicities and recorded sizes the
resultant image pattern could take a number of forms. The image
could consist of two sets of images displaying a slightly different
size or number across the array if the relative periodicities are
very close. Alternatively the image patterns could display markedly
different characteristics for example a few large images A
displayed with a large number of small images B, if the lens arrays
are of very different periodicities (say 50% different) with the
small images B displaying a much faster rate of change and movement
with lens twist and view angle. Using this concept it would also be
possible to record the formative images A and B separately and then
print the images down in separate colours to form a similar effect
where two images of different colours display different moire
effects and markedly different angular rates of change with the
smaller images with a small repeat pitch changing more rapidly with
viewing angle.
This invention also provides for another technique where two
microimage patterns are again laid down but form an image again
with different moire repeats in order to make copying and
counterfeit more difficult. In this case both images originate from
the same lens array. Both images A and B are then used for
different print runs on a document possibly but not essentially
using inks of different colours. During printing one array is
deliberately twisted out of alignment relative to the other array
so that the printed security device consists of two arrays of the
same pitch but rotated relative to each other. The same effect
could be obtained in a more controlled way by using one master
recording with the recorded lens array twisted between exposures.
The single recording would provide tight control on relative moire
pitches and image sizes, the double recording double print would
provide for a colour differential. In this case when viewed with an
analysing array either separately from the document or affixed to
it the two arrays display images of different sizes and moire
repeats. Depending on the twist mismatch angle the differences in
image sizes and repeats between the composite images could be large
or small, for example a few large image A's combined with a large
number of small image B's that would move much more rapidly with
tilt.
Another way to make copying or counterfeiting the devices more
difficult would be to slightly alter the microlens shapes from near
spherical lenses to astigmatic lenses which would have different
focal lengths along two different perpendicular axes about the
optical axis of the lens. Previously we detailed a microlens
manufacturing technique where a photoresist plate coated with a
known resist thickness was exposed to a regular rectangular grid
pattern or hexagonal pattern (etc). The exposed areas were
developed to leave islands of resist and then the resist melted to
form substantially spherical lens structures under the influence of
surface tension on the liquid meniscus which forms a spherical
surface structure. If this technique is slightly altered to use
rectangular or elliptical resist areas, then the resist islands
will be longer in one axis and when melted would then form surface
structures with different radii of curvature along two
perpendicular axes. (For this technique to be usable with other
aspects of this invention it is important that the pitches between
lenses remains constant for both axes). This method would thus form
an array of astignatic spherical lenses on a constant pitch grid
but with different focal lengths along different perpendicular
axes
with shorter focal lengths along small axes. Such astigmatic lenses
could be used during imaging with predistorted artwork to
compensate for the aberrations to form more secure images. This
would make the security devices difficult to back engineer using
reader lens arrays available in the field.
Another way to make copying and counterfeiting more difficult is to
use coloured lens arrays. Most processes to back engineer lenses
would require exposure through the lens array using blue or green
light onto a photoresist or similar material to form a printing
plate. If the analysis lens array is coloured red and effectively
absorbs blue and green light this will make back engineering much
more difficult. Likewise covering the analysis lens array with a
matrix or set of stripes of red, green, blue would allow images to
be visualised in an overlap of colours using white light but would
defeat back engineering which would require using substantially
single colour light to image through the lens onto high resolution
film or photoresist/photopolymer. In this case substantial parts of
the spectrum would be blocked by the filter stripes forming only
partial images corresponding to the unblocked areas. This would
make counterfeit back engineering the microimages using readout
lens arrays prohibitively difficult or impossible by ensuring that
the counterfeit image quality would be dramatically degraded. A
particularly effective and preferred route of permanently colouring
the lens array with a red, green, blue set of filters would be
print this onto the lens array using dye diffusing inks so that the
coloured dyes penetrated some distance into the plastic. The filter
could therefore not be removed from the lens array without
destroying it.
It is also possible to obtain other kinds of effects to make the
device more difficult to copy or counterfeit by using lens arrays
consisting of different lens types on different pitched arrays. For
example, if two sets of microimages, A and B are recorded through
two corresponding lens arrays (A,B) of different "pitches", array A
will only register to and replay through corresponding lens array A
and array B will only register to and replay through corresponding
lens array S. Here lens arrays A and B would be of the same focal
length but have different spacings. The microimage arrays A and B
are printed interspersed in the same document area. Prior art
cylindrical lens based systems would need exact register for the
image to be viewable but this approach can use typical printing
register (i.e. not within 10 microns or 10's of microns but say
within 0.5 mm). The device would be viewed through a lens array
that consists of combinations of lens arrays A and B (i.e. areas of
A and of B interspersed over small areas either randomly or
discretely). If now for example lens arrays A and B have different
focal lengths then images A and B will be perceived to move at
different apparent rates on tilting, so providing a visual effect
where for example, image B moves much faster (e.g. two image
repeats to one) than image A. Sets of microimages A and B could
also be put in different print colours so that the final image
would be a switching effect between say red A's and blue B's, with
A's and B's moving at very different apparent rates on tilting.
The advantageous characteristics of all these approaches detailed
above are as follows . . .
Exact registration of print and microlenses is not needed, unlike
other switching effects using cylindrical microlenses which require
precise registration to be effective.
The combined system would be very difficult to reproduce as a
combined lens array and matched microimage print provides security
value.
Aspects of the invention that use two different print e.g. ink
workings still do not have tight register constraints on these
elements or on their relative register to the microlens array.
One particularly advantageous approach to forming a document OVD
(optically variable device) by entirely printing techniques apart
from the creation of the initial plates is outlined below. First
print down the micro-image arrays using one or more printing ink
colours as appropriate to the technique being used. Print processes
suitable for this type of resolution would for example be litho and
intaglio printing. Then as a second stage form the overlaid
microlens array: a preferred method would be to use an intaglio
printing plate as an embossing would to form the lenses. Intaglio
printing is a well known process by which raised profiled printed
ink structures can be formed to provide a characteristic tactile
level of security on documents of value. This process involves
specialised inks and specialised print processes using engraved
printing plates and high pressures to mould ink into a relief
structure on & document. In this case the intaglio printing
plate in the intaglio print process would consist of the negative
surface relief profile of the spherical lens array required. The
spherical lens array could then be formed in situ on the document
by laying down a relatively thick layer of clear ink or varnish
over the microimage area and then using the intaglio press to mould
the lens array into this ink or varnish analogous no that used to
form intaglio printed areas on documents. In this way the entire
microlens plus microimage security device could be formed using
printing processes alone.
It should be noted that the key advantage this technique has to
offer over previous techniques based on cylindrical lenses is that
the use of spherical microlenses and corresponding matched arrays
of microimages permit relatively wide tolerances. In particular the
moire "beat" pattern of images formed by the interaction of the
lens array and the corresponding microimage array can be utilised
to allow further degrees of wide tolerance register by deliberately
using a device displaying multiple moire generated images.
Another method of mass producing image arrays instead of by ink
printing is to form the image array by selectively removing areas
from an aluminium film to form images. For example, etched micro
print of this form is often incorporated into window thread
metallic strips added as a thread to security paper such as those
used for banknotes and other high value documents. These images are
normally formed on an aluminised plastic film by either printing on
an enchant solution to remove selective areas of aluminium to form
images, or printing a protective layer on the aluminium then
removing unprotected areas using an etch solution. Using either
technique one could organize that an aluminium or other metal film
in a paper thread carried an array of microimages by using a
microimage plate for the initial printing. This would then enable a
security paper window thread to carry a covert microimage
revealable by overlaying it with a separate corresponding microlens
array. One would also anticipate a process of producing a security
device consisting of an integrated microlens array and
corresponding image array to form a directly viewable security
device by a process of embossing a microlens array relief profile
into a suitable material (e.g. polyester), aluminising and then
forming microimages in the aluminium layer by a print/etch process
as outlined above to form a microlens security device that would be
integrated into paper products as a window thread.
One would also use this type of process for forming a microimage
array into the type of dot/half toned aluminium areas used to form
the reflective layer in de-metallised holograms used for security
overlays and overlaminates for example to protect photographs as is
known elsewhere. So for instance a secure substantially transparent
holographic overlaminate containing a holographic OVDeffect could
also contain a covert microimage pattern formed within the shape or
layout of the dons of aluminium forming the partially metallised
layer which would be revealed using a microlens array overlay.
Alternative methods of integrating the lens array onto a document
would include embossing or forming the lens array into a UV curable
resin applied over the microimages (a process that would use
considerably less pressure than intaglio print) and embossing the
spherical lens arrays separately using conventional embossing
equipment and transferring the embossed lens film to the document
as a label or using hot stamping techniques.
For documents or plastic transaction cards and the like where the
substrate is plastic there are other useful embodiments of this
invention which enable the microlenses and corresponding
microimages to be incorporated together. In this case the
microlenses could be incorporated into one surface of a
substantially optically clear plastic substrate, whilst the
thickness of the substrate would provide the necessary separation
from the microimage array which would be placed on the other side
of the substrate (or within an inner layer of the substrate for the
case of thicker plastic cards) and viewed through the substrate
material. This would avoid the need for a raised area of
microlenses as needed with documents and would enable the
microlenses to be combined within the substrate either by using an
embossing process before manufacture or during manufacture.
BRIEF DESCRIPTION OF THE DRAWINGS
Some examples of security devices according to the invention will
now be described with reference to the accompanying drawings, in
which:
FIGS. 1A and 1B illustrate respectively in cross-section and in
perspective view a security device when viewed normally;
FIGS. 1C and 1D are similar to FIGS. 1A and 1B showing the security
device when viewed through a microlens array viewer;
FIG. 1E is a schematic, perspective view of the arrangement shown
in FIGS. 1C and 1D;
FIGS. 2A and 2B illustrate the recording and replay of a pair of
images respectively;
FIGS. 3A and 3B illustrate a document carrying a security device,
and the document when viewed through a microlens array
respectively;
FIGS. 4A to 4C illustrate an example of the production of a
security device;
FIG. 5 illustrates a further example of a security device; and,
FIG. 6 illustrates a security document holder.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIGS. 1A and 1B illustrate a substrate 1 carrying a two-dimensional
array of microimages 2 (only one set of regularly spaced
microimages being shown for clarity). The substrate 1 may comprise
a security document, such as a banknote, or a separate substrate
which is subsequently adhered via an adhesive (not shown) to a
security document. Each microimage 2 within the array is
substantially identical and typically has a dimension of up to 250
microns. Each microimage comprises an image with detail which can
be resolved to say 5 or 10 microns. Typically, there will be many,
e.g. mode than one hundred microimages provided per array.
To the naked eye, the image recorded in each microimage will be
unrecognisable as illustrated in FIG. 1B. However, when a matching
array 3 of spherical microlenses is brought into alignment with the
microimages as in FIGS. 1C and 1E the images will replay optically
through the corresponding lenses when viewed in a direction 21 to
generate magnified images as schematically shown in FIG. 1D. Mere
the image recorded in each microimage within the array is the
letter "A".
The viewer would need to align the analysing lenses 3 for focal
length (i.e. planar separation) to bring the microimages 2 into
focus and then correct for tilt and twist. The viewer will first
see, as the analysing lenses are twisted into place, an array of
many small images which gradually reduce in number to a few large
images when the alignment is perfect.
This is effectively the visual equivalent of viewing a normally
printed image such as a photograph. The effect can be used to
provide an optical security device, as counterfeits will be readily
detected.
In the FIG. 1 example, each microimage 2 is identical. It is also
possible, however, to record sets of microimages which define
different images or different views of the same image in .order to
achieve a three-dimensional image and/or switching or moving replay
effects similar to those produced by holography. This is
illustrated in FIG. 2. In this case, each lens of a lens array 4
records the different view point of an object I1 onto the
photosensitive coating 5A on a substrate 5B. Object I2 is then
recorded from a different angle. In this example, the different
objects I1,I2 are the letters "A" and "B" respectively. When the
array printed substrate 5 is viewed through a similar microlens
array 4' (FIG. 2B) depth and movement effects will be visible as
the viewer changes viewpoint from X to Y: the viewer will see I1 at
position X and I2 at position Y.
FIG. 3 illustrates an example of a security document 6
incorporating a security device 7 similar to the devices shown in
FIGS. 1 and 2. As indicated in FIG. 3A, the image used in the
microimage array in the security device is not resolvable to the
naked eye. The array will have typically been printed onto the
substrate, such as paper, using standard ink printing techniques
such as lithographic printing.
In order to inspect the security device 7, a microlens array 8 is
laid over the security device 7 causing the microimages to
reconstruct and generate magnified images as shown in FIG. 3B.
Typically, the microlens array 8 will be provided in a reader
device (not shown) into which the document 6 is inserted.
To create a hidden security print feature as in FIG.3 typically
requires the creation of a master printing plate consisting of an
array of microimages--preferably formed by imaging one or more sets
of graphics down onto a high resolution photosensitive recording
medium via a microlens array.
This results in a regularly spaced array of microscopic images of
minimum detail size around 5.mu., and typically of maximum image
size 100-200.mu.. These images and the detailing would not be
viewable by unaided eye. This pattern could also be created by
using direct write photomask manufacturing techniques as used in
the semiconductor industry.
The printing of the microimage pattern on the plate onto a security
document or plastic card, etc., would use standard security
printing methods e.g. lithographic or intaglio printing which are
capable of producing high resolution printing detail to as small as
5 to 10 microns.
This would then form a security document comprising a covert array
of microscopic images unrecognisable to the unaided eye and so
small as to be substantially non-discernible using standard hand
magnifiers, etc. This array of microimages may be revealed by
overlaying it with the array of microlenses 8, probably supplied as
an embossed transparent plastic sheet which has an array of surface
relief microlenses. When the viewing sheet is properly aligned the
eye would see a vastly magnified image of the original microscopic
image in the array, which would exhibit characteristic depth and
movement effects. Using slightly more sophisticated multiple
exposure techniques at the origination stage would also allow this
image to "flip" or switch between two or more different graphical
images, coloured images or lines to give an optically variable
switching effect on tilting the device during inspection for
authenticity.
One method of manufacturing a security device is shown in FIG.
4.
An array of one or more sets of microimages 11 (FIG. 4A) would be
printed onto a security document substrate 10 using conventional
printing technology such as intaglio printing.
The print 11 is then overlaid with a transparent polymeric resin
material 12 (FIG. 4B) and then a corresponding array of microlenses
13 is formed either e.g. by embossing with a stamper, or using a UV
curable composition or by using a casting/curing process in situ
(FIG. 4C). Alternatively an array of pre-formed microlenses could
be applied over the print using transfer techniques as currently
used in the holographic and print industry (e.g., hot stamping
foil).
However, the preferred route would be to form the corresponding
microlenses in situ either by moulding the transparent plastic with
an uninked intaglio plate on a printing press, or by a casting
approach. This would involve overcoating the print feature 11 with
a transparent curable material 12 and then forming the microlens
array into the surface of this transparent material using an
uninked intaglio printing plate or an embossing mould 14 and then,
after removing any solvent, curing the material using UV light,
electron beams or similar techniques to polymerise and cure the
layer. Optionally the microlenses may be replaced by a diffractire
microlens (i.e. a holographic optical element) applied or formed by
the same in-situ moulding process or applied as standard hot
stamping foil or similar.
FIG. 5 illustrates another approach applicable to plastics
substrates in which microimages 15 (one shown for clarity) are
printed on one side of a plastics substrate 16 which is transparent
while microlenses 17 are cast or moulded into the other side of the
substrate 16. The substrate thickness is used as the optical spacer
required to allow the image recorded in the microimages to be
recognisable through the lenses. The substrate in other areas may
be provided with an opacifying coating 17 which could then be
printed over with security indicia.
FIG. 6 shows a document holder 22 to enable a security device
easily to be authenticated, The security document holder 22 has an
arrangement 25 such as a slot or spring loader assembly for
locating a security document 23 having a microimage security device
24 substantially in register with a set of microlenses 26 located
within the document holder such that the security device can be
viewed through the corresponding microlens array 26 to form a
magnified image 27 for verification by an observer 28.
* * * * *